It has previously been recognized that certain woods, such as red cedar, redwood, southern yellow pine and others, contain tannin and a high percentage of water-soluble natural coloring matter, such as tocopherol, which have a tendency to bleed through an applied primer coat and into the finish coat of water-base paints. This tendency has detracted from the more extensive commercial use of water-base paints on these woods, despite the many recognized advantages making these paints otherwise desirable for use in exterior finishes as primers and/or top coatings.
It has previously been proposed to use basic silicate of white lead as stain-blocking agent to prevent the aforesaid tendency of color bleeding through the primer coat applied to these unpainted woods and into the finish coats. See, for example, F. J. Williams et al., Industrial and Engineering Chemistry, Vol. 40, pages 1948 et seq. (1948). Improved paint compositions containing such lead compounds and tung oil are described in U.S. Pat. No. 3,214,398. Despite their effectiveness as stain-blocking agents the use of lead-containing compositions is contraindicated, however, because of their toxicity. Moreover, recent U.S. government regulations limit the amount of lead that may be used in paint, which may curtail or nullify the use of lead compounds as stain-blocking agents.
More recently, modified barium metaborate has been described in the literature for use as a stain-blocking agent. See R. T. Ross, American Paint Journal, 55 (37), Mar. 1, 1971. This compound has presented formulation difficulties in paint manufacturing; for example, because of the difficulty of achieving and maintaining uniform dispersion of the metaborate during the time the paint is stored and prior to its sale for use.
Certain wood stain-blocking agents currently marketed, such as those comprising nonfilm-forming acrylic polymers, and those comprising calcium phosphosilicate are relatively expensive because these require extremely high concentrations when applied as a constituent of the stain-resistant paint (1 pound or more per gallon of paint).
Also suggested as overcoming the problems of staining and discoloration of water base paints applied as a finishing coat to certain woods, is the use of guar gum and/or tartar emetic as a tannin precipitant, incorporated in a water-base primer paint together with a dye mordant to fix the natural coloring matter in the wood U.S. Pat. No. 3,438,914). The tartar emetic in and of itself may serve as the mordant or certain alums or normal aluminum salts, such as sulfate or phosphate, may be so employed. The described additives are recommended for use in oil emulsion paint compositions, which, it is indicated, are slightly acidic when formulated. While the incorporation of these additives in latex paint compositions is stated to provide suitable resistance to staining acid discoloration when applied to wood surfaces, the use of these in latex compositions presents certain problems not encountered in oil emulsion paints. Among these mentioned drawbacks is the alkaline pH of water-base latex paints as formulated, which must be brought to slightly acid or neutral pH to avoid darkening of the tannins and dyes in the wood and to enable the tannin precipitant to function effectively.